Semiconductor light emitting devices are available for emitting light at predetermined wavelengths. In particular, blue and blue-green wavelengths are of special interest for their use in full-color displays, as read-write laser sources for high density optical information storage, and as sources for undersea optical communications.
Extensive research has been devoted to semiconductor light emitting devices including Group II-VI materials to produce blue light. The Group II-VI compounds, ZnSe and ZnSSe, in particular, have received considerable attention as materials for fabricating a blue light emitting device. In addition, a blue light emitting device may also be fabricated from the solid solutions of (Zn,Cd)(Se,S) and, since these materials form solid solutions for varying compositions, a wide variation of optical emissions wavelengths could be achieved.
An article by Ren et al., for example, entitled ZnSe Light-Emitting Diodes, appearing in Appl. Phys. Lett. 57 (18) Oct. 29, 1990, discloses a ZnSe p-n light-emitting junction including chlorine as the n-type dopant and lithium as the p-type dopant. U.S. Pat. No. 5,045,894 to Migita et al. entitled Compound Semiconductor Light Emitting Device discloses a strained layer superlattice light emitting device including ZnSe as one of the layer materials. Another article by Ren et al., entitled Blue (ZnSe) and Green (ZnSe.sub.0.9 Te.sub.0.1) Light Emitting Diodes, appearing in the Journal of Crystal Growth, 111, pp. 829-832, (1991), discloses both a ZnSe p-n junction and a double heterojunction ZnSeTe device. See also, for example. Jeon et al., "ZnSe Based Multilayer PN Junctions as Efficient Light Emitting Diodes for Display Applications", Appl. Phys. Lett. 60 (7), Feb. 17 1992; Haase et al , "Blue-Green Laser Diodes", Appl. Phys. Lett. 59 ( 11), Sept. 9, 1991; Xie et al., "Blue/Green PN Junctions Electroluminescence from ZnSe-Based Multiple Quantum-Well Structures", Appl. Phys. Lett. 60 (4), Jan. 27, 1992; Xie et al., "Room Temperature Blue Light Emitting P-N Diodes from Zn(S,Se)-Based Multiple Quantum Well Structures", Appl. Phys. Lett. 60 (17), Apr. 20, 1992; and Jeon et al., "Blue-Green Injection Laser Diodes in (Zn,Cd)Se/ZnSe Quantum Wells", Appl. Phys. Lett. 59 (27), Dec. 30, 1991.
Ideally, a ZnSe or ZnSSE p-n junction would satisfy most of the blue or blue-green optical emission requirements. Unfortunately such light emitters have experienced poor controllability, and/or low output power and low luminescence efficiencies. There continue to be materials limitations on the controllability of substitutional doping of ZnSe or ZnSSE and other Group II-VI materials in general. Moreover, p-type doping of ZnSe or ZnSSE is difficult and often not reproducible. In addition, good ohmic contacts to such p-type materials may also be difficult.
Other materials have also been explored for fabricating a blue light emitting device. For example, U.S. Pat. No. 5,117,267 to Kimoto et al. entitled Semiconductor Heterojunction Structure discloses a light emitting device including two layers of p-doped diamond with an intervening layer of n-type cubic Boron Nitride (cBN). While diamond is a preferred material for many semiconductor applications; unfortunately, diamond is an indirect bandgap semiconductor and thus will have poor optical emission efficiency. Moreover, n-type doping of diamond for a heterojunction device is not presently readily or reproducibly achievable.